Generally, an orthogonal frequency division multiplexing (hereinafter abbreviated OFDM) communication system is a sort of a communication scheme used by various systems (IEEE 802.11a/g, HiperLAN, IEEE 802.16, DSL, DAB, DVB, etc.). The OFDM scheme is very efficient for selective-fading of a communication channel. In the OFDM system, a plurality of sub-carriers are used, whereby the selective-fading is regarded as flat-fading. And, it is advantageous that a scheme for compensating for fading in an overall system becomes simplified.
To obtain the above features, an OFDM system introduces modulation/demodulation through FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform). Data to be transmitted is preferentially allocated to each sub-carrier. A signal in a time domain is then obtained by performing IFFT on the allocated data. A receiving end performs FFT on a received signal, estimates a channel through a pilot sub-carrier set in transmission, and then extracts data from the signal.
The OFDM system has had a problem that a peak power to average power ratio (PAPR) gets increased like a code division multiplexing access (CDMA) system. In particular, if a size of an OFDM symbol grows, this problem gets more serious. To correct the problem, various PAPR schemes have been proposed. Most of the PAPR schemes deal with the schemes by data manipulations in frequency domain. In the related art PAPR schemes, an optimal PAPR is found by repeating a process for manipulating data in frequency domain and checking PAPR in time domain. In this process, it is disadvantageous that complexity in producing a large OFDM symbol should be repeated each time.
The OFDM modulation scheme is able to employ multiple-users and its implementation is generally called OFDMA (OFDM multiple access) scheme. And, it is able to implement the OFDM modulation scheme by distributing sub-carrier within one large OFDM symbol to different users. Each of the different users should receive corresponding data by demodulating sub-carrier channels allocates to them, respectively. For this, a signal can be generally found by demodulating the large OFDM symbol entirely. This work having nothing to do with the number of sub-carriers distributed to the corresponding user wastes power of the corresponding user and needs hardware of high performance.
FIG. 1A is a diagram of a process for producing an OFDM signal in a transmitting side according to a related art, and FIG. 1B is a diagram of a process for receiving and recovering an OFDM signal transmitted from a receiving side to a transmitting side.
Referring to FIG. 1A, after IFFT having a size of N (N-size) has been performed by converting input data symbol {right arrow over (d)}=[d0, d1, . . . , dN−1]T to serial-to-parallel (S/P), parallel-to-serial (P/S) conversion is performed. Equation 1 describes a scheme of Inverse Fourier Transform.{right arrow over (s)}=[s0, s1, . . . , sN−1]T=F−1{right arrow over (d)}  [Equation 1]
Here, F indicates a Fourier transform matrix. A cyclic prefix (CP) is inserted in a vector {right arrow over (s)}, modulated into sub-carrier frequency and then transmitted via an antenna.
A signal resulting from removing a CP from a signal received from a transmitting side by a receiving side can be represented as Equation 2.{right arrow over (r)}={right arrow over (h)}{right arrow over (s)}+{right arrow over (w)}  [Equation 2]
In this case, {right arrow over (h)} is a response vector of channel and {right arrow over (w)} corresponds to reception noise. Vector {right arrow over (r)} is converted to serial-to-parallel (S/P) and demodulated by FFT. A Fourier-transformed signal is represented as Equation 3.{right arrow over (v)}=F{right arrow over (r)}=H{right arrow over (d)}+F{right arrow over (w)}  [Equation 3]
In this case, if a channel is estimated, a transmission signal can be demodulated according to Equation 4.{right arrow over (d)}E=(HHH)−1HH{right arrow over (v)}  [Equation 4]
The processes, as shown in FIG. 1A and FIG. 1B, are adopted by most of the OFDM systems.
A multiple access structure and complexity of a PAPR enhancing method according to a related art are explained as follows.
First of all, in order to configure a multiple access structure, each user should know what kind of sub-carrier is allocated to himself. Corresponding data to be transmitted to the corresponding user is allocated to a position of the sub-carrier allocated to the corresponding user and data to be transmitted to all users gather together to form a data vector {right arrow over (d)}. And, by producing a time domain signal according to Equation 1 and executing Equation 4 in a receiving side, estimated values for all data should be obtained. Subsequently, each of the users undergoes a process for extracting a data value from the position of the sub-carrier allocated to the corresponding user. In this process, each of the users should go through Equation 3. So, demodulation should be always performed with complexity of Nlog2 N.
Schemes to enhance PAPR mostly use the expression of Equation 5.{right arrow over (d)}P=MSMP{right arrow over (d)}  [Equation 5]
In this case, MS is a matrix to transform a phase component of a data vector {right arrow over (d)} and MP is a permutation matrix to rearrange an order of the data vector {right arrow over (d)}. So, PAPR is found after {right arrow over (d)}P transformed according to Equation 5 has been converted to a time domain by Equation 1. In general, in order to make PAPR attenuate, signals in time domain are found using various combinations of MS and MP and the signal having the best performance is then searched for. So, in order to execute PAPR enhancement in frequency domain, the transform according to Equation 1 should be always used and complexity of Nlog2N is added each transform according to Equation 1.
However, in the related art OFDM system, since the complexity, which should be modulated/demodulated by each user to implement the multiple access environment, is equal to the overall OFDM symbol size regardless of the number of sub-carriers allocated to the corresponding user, lots of calculations are assigned to the corresponding user. So, more battery losses are inevitable and hardware of high performance is required.
And, in the related art OFDM system, various schemes to solve the PAPR problem depend on data manipulations in frequency domain. And, it is necessary to execute the process of checking PAPR of a real transmission signal by executing IFFT each time to check the performance after completion of the data manipulation. In doing so, a large OFDM symbol has to be modulated each time to need lots of calculations. So, more battery losses are inevitable and hardware of high performance is required as well.